1. Field of the Invention
This application relates to interference cancellation in an antenna receiver, specifically as such interference cancellation applies to wireless telecommunications.
2. Description of the Related Art
One of the major problems that limit performance in cellular networks today is self-interference, both co-channel and adjacent channel. One of the main reasons that interference is such a problem is that cellular operators are trying to maximize the spectral efficiency of their networks by decreasing the reuse factor. The reuse factor defines how often the same radio channel (frequency) is reused within a group of cell sites and sectors, and thus, the smaller the reuse factor the more radio channels per site/sector, which increases the spectral efficiency. Unfortunately, as the reuse factor is decreased, the amount of interference is increased, because the physical distance between the receiver of interest and the interference is decreased.
There are a number of methods that have been developed for canceling, suppressing, or otherwise mitigating interference in a cellular network. One of the more common techniques used at a base station is to employ two receive antennas for the purposes of interference cancellation. There are a number of signal processing algorithms that have been developed for base station applications including Interference Rejection Combining (IRC) and Two-Branch Intelligent Antenna (TBIA). See for example, “A System Performance Evaluation of 2-Branch Interference Rejection Combining”, VTC 1995, S. Craig, et al.; and “Intelligent Antennas for Wireless Communications—Uplink” Bell Labs Technical Journal, R. Buehrer, et al. These techniques have been applied to TDMA and GSM cellular networks. The use of two or more antennas provides receive diversity since the antennas provide separate, “diverse” channels, through which the transmitted signal propagates. With two antennas, the receiver is said to have a diversity order of L=2. If these two channels exhibit a sufficient amount of de-correlation then the aforementioned signal processing algorithms can be employed to cancel a certain amount of interference, and also provide gain in noise-limited conditions as well.
Although the use of two receive antennas is generally applicable at the mobile terminal as well, there are cost, space, and aesthetic issues, which to date, have prevented wide-scale deployment, especially in the traditional mobile terminal. Because of these issues, there has recently been significant effort expended to develop techniques that suppress interference using just one receive antenna, particularly for GSM voice and packet data services. See for example, “Single Antenna Interference Cancellation (SAIC) for GSM Networks”, VTC, 2003, A. Mostafa, et al. These techniques are collectively referred to as Single Antenna Interference Cancellation (SAIC).
The GSM circuit-switched voice service and the first-generation, packet-switched data service, referred to as GPRS, employ Gaussian Minimum Shift Keying (GMSK) modulation. GMSK is a binary modulation scheme, which implies that each input bit produces a modulator output symbol. In addition, GMSK is also a single-valued modulation scheme, which implies that two signal constellation points take on only one value suitably modified by +−1, and that these points lie on the same axis. Single-valued modulation schemes such as GMSK and Binary Phase Shift Keying (BPSK) have a unique characteristic in that they can be split into two virtual paths at the receiver by processing of the real and imaginary parts. When a co-channel or adjacent channel interferer is present that traverses a different channel than the desired signal, a receiver can exploit these two virtual paths to achieve an effective diversity order L=2. “Co-channel Interference Cancellation Within the Current GSM Standard”, Proceedings of the IEEE 1998 International Conference on Universal Personal Communications, Oct. 5-9 1998, Florence, Italy, H. Trigui, et al., discusses this methodology. The two paths can be processed using conventional signal processing algorithms for interference cancellation.
The amount of gain achievable with two receive antennas for a typical cellular uplink, or the two virtual paths created by suitable processing of the GMSK signal for the downlink, is limited by the amount of correlation between the paths. The correlation value is typically defined as the correlation coefficient of the signal envelopes. The smaller the correlation, the greater the gain, and conversely, the greater the correlation the smaller the gain. In the limit, the maximum gain is achieved when the paths are identically and independently distributed (i.i.d.), which corresponds to a correlation of zero. Unfortunately, the maximum gain has not been realized in practice. As known in the art, a typical correlation value for two cellular base station antennas is approximately 0.7. Thus, the gain is decreased from the maximum achievable. To achieve lower levels of correlation requires greater spatial separation of the two receive antennas, which is limited by the space available, both at the base station and the mobile terminal. However, decreasing correlation beyond what is achievable today is not typically an option, so one must look for other methods to increase the interference cancellation gain.